1. A pneumatic tire for vehicles comprising a tread, an annular reinforcing structure incorporated in said tread, two sidewalls comprising eleastomeric material and terminating into beads intended to be encased in appropriate seats in the wheel rim, said sidewalls having in those sections of each plane containing the axis of rotation of the tire, a section midline whose convexity is directed towards the tire inside, said convexity being unchanged in sense under the thrust of tire inflation pressure, said annular reinforcing structure being substantially inextensible and defining contours, in proximity of the lateral edges of said tread, whose points are at a substantially invariable distance, with respect to pressure variations, from the points defined by said beads in the same section planes, characterized in that said annular reinforcing structure, in the sections of each plane containing said axis of rotation, when the tire is inflated at its service pressure, has a profile whose curvature has its own concavity, at the structure''s central portion, directed radially inwardly of the tire, while the concavity of the profile at the structure''s lateral portions is directed radially outwardly of the tire.